1. Field of the Invention
The present invention relates to a relay node and a relay method for relaying a radio signal using a multiple-input multiple-output (MIMO) system.
2. Related Background Art
In recent years, a MIMO multihop system, which is a combination of a multihop system and a MIMO system, is getting attention as a technology for relaying radio signals. In the multihop system, signals are transmitted from a source node, which is the source of the signals, to a destination node, which is the destination of the signals, via 1 or more relay nodes located between the source node and the destination node. This system has advantages that the range where signals can be received is expanded by relaying signals, that the signal transmittable areas are not theoretically limited, and that a wireless network can be established quickly. The MIMO system, meanwhile, is a communication system in which multiple transmitting antennas and receiving antennas are used to transmit and receive signals in order to increase the channel capacity through efficient use of a wireless space.
Regarding the MIMO multihop system, H. Shi, T. Abe, T. Asai, and H. Yoshino, “A Relaying Scheme using QR Decomposition with Phase control for MIMO wireless Networks,” Proc. IEEE International Conf. On Communications Seoul, May 2005, p. 2705-2711, H. Shi, T. Abe, T. Asai, and H. Yoshino, “Relay Techniques in MIMO Wireless Networks,” Proc. IEEE Vehicular Technology Conf., September 2005, p. 2438-2443, Japanese Unexamined Patent Application Publication No. 2006-148867, and Japanese Unexamined Patent Application Publication No. 2006-148868 disclose a technology of computing a transmission weight matrix on the basis of a channel matrix between the source node and the destination node in the relay node, and then multiplying a received signal by the transmission weight matrix to improve the diversity gain between the relay nodes and between the multiple antennas in each relay node. Specifically, “A Relaying Scheme using QR Decomposition with Phase control for MIMO wireless Networks,” and “Relay Techniques in MIMO Wireless Networks” proposes QR-P-QR, QR-P-ZF, and ZF-P-QR as such system. QR-P-QR is a system for performing QR decomposition on a backward channel on the source node side and a forward channel on the destination node side. QR-P-ZF is a system for performing QR decomposition and then zero-forcing (ZF) on the backward and forward channels. ZF-P-QR is a system for performing zero-forcing and then QR decomposition on the backward and forward channels.
However, if the ratio between transmission quality of the forward channel and transmission quality of backward channel in each relay node (e.g., SNR ratio) changes, a different relay method should be used in order to achieve the maximum channel capacity. Therefore, if all of relay nodes uniformly select a relay method when they have different channel transmission quality, it becomes impossible to ensure that an appropriate relay method is always selected in the entire communication system, causing reduction in the channel capacity.